Howardites
Group · Stony meteorites › Chondrites › Ordinary chondrites

H chondrites

High-iron ordinary chondrites: about a third of all meteorites seen to fall.

H chondrite
Classification
Stony meteorites › Chondrites › Ordinary chondrites › H chondrites
Share of witnessed falls
about 33.8%
In our catalogue
18,126 meteorites, 353 seen to fall
Heaviest
Jilin, 4 t
Earliest recorded fall
Vago, 1668
Most recent fall
Huaxi, 2010

H chondrites have the highest total iron of the ordinary chondrites, with about 15% to 20% iron-nickel metal by weight, so they are strongly magnetic and feel heavy. Their olivine is relatively poor in iron (about 16–20 mol% fayalite).

They are about 34% of falls. Many H chondrites have cosmic-ray exposure ages clustered around 7 to 8 million years, showing that a large share of them were knocked off their parent asteroid in a single collision.

The asteroid 6 Hebe has been suggested as their parent, though this is not certain.

Mineralogy and chemistry

Olivine, low-calcium pyroxene (bronzite) and metal are the main minerals, with plagioclase, troilite, chromite and phosphates in smaller amounts. Total iron is about 27% by weight, and roughly two-thirds of it is in metal. Chondrules are small for an ordinary chondrite, averaging around 0.3 mm. Low-calcium pyroxene contains about 14–18 mol% ferrosilite, the iron-rich end member, and the olivine about 16–20 mol% fayalite.

Oxygen isotopes put H chondrites slightly above the terrestrial line, with a Δ17O value (the offset from Earth's line) of about +0.7 per mil, lower than L and LL. H5 is the most common petrologic type, followed by H4 and H6; unequilibrated H3 chondrites are rare.

Parent body and formation

Metallographic cooling rates and radiometric ages from H chondrites fit an asteroid perhaps 100 to 200 km across that heated from inside and cooled for tens of millions of years, with the most strongly heated rock at the centre. Later impacts broke and stirred it, so many H chondrites are regolith breccias with solar-wind gases picked up on the surface.

Cosmic-ray exposure ages show how long a meteorite travelled as a small body in space. Many H chondrites cluster at 7 to 8 million years, and a second cluster near 33 million years, each marking a collision that sent a swarm of fragments towards Earth. The young Karin cluster in the Koronis asteroid family, formed about 5.8 million years ago, has been proposed as a recent source, while 6 Hebe has long been suggested as a parent. The IIE irons share their oxygen isotopes and may come from the same body.

Abundance, type specimen and famous examples

H chondrites make up about a third of witnessed falls and a large share of finds. Ordinary chondrite groups are named for their chemistry, so there is no type specimen. Jilin (H5), which fell in China in 1976, produced the largest single stone known, 1,770 kg. Weston (H4), Connecticut, 1807, was the first fall in the United States to be studied scientifically. Pultusk (H5), Poland, 1868, fell as tens of thousands of stones; Forest City (H5), Iowa, 1890, and Gao-Guenie (H5), Burkina Faso, 1960, were also large showers. Košice (H5) fell in Slovakia in 2010, and Zag (H3–6), Morocco, 1998, contains tiny salt crystals with trapped liquid water.

How to recognise one

An H chondrite feels heavy and grips a magnet firmly. Cut and polished, it shows plentiful small metal grains evenly scattered through grey rock, more than in an L or LL. Chondrules may be crisp in type 4 and blurred in types 5 and 6. Weathered finds turn reddish brown as the metal rusts. Confirming the group needs analysis of the olivine.

Famous examples

Related groups

See all 18,126 H chondrites

Photos shared under Creative Commons licences (Wikimedia Commons).